1. Field of the Invention
This invention relates to a curtain coating method and a curtain coating apparatus for uniform application of coating solutions and, more particularly, to the method and the apparatus for curtain coating solutions on to a continuously travelling strip like support (hereinafter referred to as "web") which is employed in the production of photographic films, photographic printing papers, magnetic recording tapes, pressure-sensitive recording papers, offset plate materials and the like.
2. Description of the Related arts
Conventionally, a curtain coating technique has been applied to the production of photographic films, photographic printing papers and the like.
First, descriptions will be given of conventional curtain coating methods as follows.
The basic arts of curtain coating are described, e.g., in U.S. Pat. No. 3,508,947 and U.S. Pat. No. 3,632,374 which correspond to JP-B-49-24133 and JP-B-49-35447 respectively (The term "JP-B" as used herein means an "examined Japanese patent Publication").
Further, S. F. Kistler discloses a theory of curtain coating in "AlChe Winter National meeting" (1982), and describes the following three phenomena which predominantly determine the coating rate in the curtain coating method:
(1) the phenomenon that fine bubbles are entrained in a gap between a web and a coating solution (This phenomenon is called "the air entrainment phenomenon" hereinafter), PA0 (2) the phenomenon that a foot-like cross-sectional shape of the impingement zone can develop a pronounced heel that can give rise to coating nonuniformity. (This phenomenon is called "the heel phenomenon" hereinafter, and it occurs in a case where a coating solution is made to flow down at a high flow rate), and PA0 (3) the phenomenon that a coating solution bounds at the web surface without adhering thereto (This phenomenon is called "the sagging phenomenon" hereinafter, and it occurs in the same case as the phenomenon (2), namely a case where a coating solution is made to flow down at a high flow rate).
As for attempts to elevate the upper limit of coating speed in this curtain coating method, there is disclosed the means of inhibiting "the air entrainment phenomenon", e.g., by applying an electrostatic field between a web and a coating solution (JP-A-62-197176). (The term "JP-A" as used herein means an "unexamined published Japanese patent application").
In recent years, however, the coating operation has been performed at a high speed of 250 m/min or above and the flowing-down rate of a curtain of coating solution has also been increased. As a result thereof, the retardation of coating speed due to "the sagging phenomenon" has come to a greater problem than the retardation caused by the aforementioned "air entrainment phenomenon".
Generally speaking, in order to get rid of nonuniformity in the thickness of the edge part of a free falling curtain, a curtain coating apparatus is usually equipped with guide plates for truing up both the edges of the coating solution on the slide plane, or edge guides for supporting both edge parts of the free falling curtain. In many cases, however, the sagging phenomenon occurs in the edge part of the free falling curtain, in which a thickness of the curtain is apt to be nonuniform.
Methods for suppressing the sagging phenomenon and techniques to improve the conventional coating techniques are disclosed in the next two references.
U.S. Pat. No. 5,393,571 which corresponds to JP-A-3-146172 discloses a method of conducting a curtain coating in which a total viscosity of coating solutions is specified and a surface roughness of web is adjusted to at least 0.3 .mu.m in order to achieve a high-speed coating in a high flow rate range beyond 4 cc/cm/sec.
U.S. Pat. No. 5,391,401 which corresponds to PCT publication translated in Japanese No. Hei6-503752 discloses a method of conducting a curtain coating stably in which a viscosity of coating solution to form the lowest layer is specified with using a pseudoplasticity solution as the coating solutions.
In these methods, the viscosity is specified in order to conduct a high-speed coating stably. However, specifying only the total viscosity or the lowest layer viscosity is insufficient for realizing the high-speed coating and the uniform coated surface condition in multilayer coating. For instance, the coating solution for the lowest layer is likely to be designed so as to have low viscosity and low gelatin concentration from the viewpoint of high-speed coating suitability, but the coating solution for an upper layer is generally made as dense as possible from the viewpoints of ease of preparation, feeding, reduction of drying load and so on, thereby resulting in a rise of gelatin concentration.
When the curtain coating method is adopted in the multilayer coating, it turned out that unevenness was caused in the coated film when there are large differences in gelatin concentration and viscosity between the coating solutions for the lowest layer and the directly upper layer. This is because, although the curtain is drawn out to be rendered thin at the times when it is formed from the slide plane and impinges on the web during the curtain coating operation, this drawn-out condition is not uniform in the direction of coating width in the case where there are large differences in gelatin concentration and viscosity between coating solutions for the lowest layer and the directly upper layer, resulting in generation of unevenness.
Next, a description will be given of conventional curtain coating apparatuses as follows.
In the steps of forming a free falling curtain, edge guides support both edge parts of the curtain formed, and various techniques thereto are disclosed. For example, Japanese examined patent publication No. Sho 58-37866 discloses a method of introducing an auxiliary liquid between the free falling curtain and the edge guides which are prepared at both the edge parts of the curtain, and Japanese examined patent publication No. Hei 6-61517 (hereinafter referred to as JP-B-6-61517) discloses a method of introducing an auxiliary liquid close to the hopper lip.
FIG. 1 is a perspective view of a basic structure of a curtain coating apparatus as an embodiment of the present invention.
A coating head 1 has a plurality of slits 12 respectively connected to manifolds 10, and the coating solutions extruded from each slit 12 flows down on a slide plane of a slide hopper 7 and falls down to form a free-falling curtain 5 of the coating solution.
Both sides of the curtain 5 are supported by edge guides 4, and the curtain 5 falls and impinges at a line on a surface of a web 3 which travels in the arrow direction while supported by a backup roller 2, so that a coated film 8 is formed.
FIG. 4 is a front view of a curtain coating apparatus to which an method disclosed in JP-B-58-37866 cited above is applied for introducing the auxiliary solution for the liquid curtain. Such an apparatus is shown as one example of conventional curtain coating apparatuses. In FIG. 4, each edge guide 4 is equipped with conduit 14 in a place situated downward at a fixed distance from the departing position of the liquid curtain from the hopper lip, and the conduit 14 is bent down so that its outlet turns to the lower side and each direction of the outlet are parallel to the falling direction of the free falling curtain 5.
FIG. 5 is a side view of a curtain coating apparatus to which a method disclosed in JP-B-6-61517 cited above is applied for introducing the auxiliary solution into the free falling curtain. Such an apparatus is shown as another example of the conventional curtain coating apparatuses. Therein, an auxiliary solution supply pipe 15 is kept upright and arranged upward at a fixed distance apart from the departing position of the liquid curtain from the hopper lip.
As shown in the above examples, the auxiliary liquid flows along each of the edge guides, so that the film formation can be stabilized and the coated film edge corresponding to the edge part of the liquid curtain is prevented from thickening.
With respect to the conventional technique as shown in FIG. 4, the conduit 14 is disposed so as to go across the edge guide 4 and project into the edge part of the free falling curtain 5. Thus, the outlet of the conduit must be bent to the lower side. As a result, it is required for the conduit to have an adequate length for changing the direction of the outlet by bending. This requirement occupies the minimum distance between the hopper lip and the outlet. Accordingly, there will be no introduction of the auxiliary solution into the section formed between the hopper lip and the outlet. If the edge thickening occurs in this section, the coating is carried out under the condition of the thick edge. Moreover, it is necessary for a bent part of the conduit to be projected into the free falling curtain 5 to cause a turbulent flow therein. As a result, there comes a problem of defection of longitudinal streaks in the coated layer.
With respect to the conventional technique as shown in FIG. 5, in taking into consideration a position where the auxiliary solution supply pipe 15 is disposed, the auxiliary solution is supplied from above to the edge of the liquid curtain in the thickness direction of the free falling curtain. In short, the auxiliary solution is supplied from outside of the free falling curtain 5. Therefore, it is difficult for the auxiliary solution to be supplied so as to go across the curtain edge in the thickness direction and uniformly reach the backside of the curtain 5. Specifically, in multilayer formation, such an uneven supply of the auxiliary solution makes a difference in edge part thickness between the upper layer and the lower layer. Moreover, it is very difficult to dispose the pipe 15 so that the auxiliary solution is supplied to the same position every time the coated layer is produced. Furthermore, since the auxiliary solution is brought into contact with a curtain surface, goes across the curtain and then reaches the edge part, the auxiliary solution is supplied from the direction in which the curtain flow maintaining pressure is weak. As a result, change in an auxiliary solution supplying pressure causes an external disturbance in the flow at the curtain surface, thereby causing such defects as longitudinal streaks.